As the Gutians, I sometimes build Armies for no apparent reason at all.

Seconded. If I place a Field and it gets converted, I have no means of converting it back, and it becomes a building spot for my enemy to invade my lands further. If I place an Army and it gets converted, I can fight back (to my financial advantage). Placing the Field gets me $1, but the Gutians have only very limited use for their money.